1. Field of the Invention
The present invention relates to a charge-pump or charge pumping circuit for boosting supply voltage by means of capacitors, and the invention also relates to a load-driving or load drive circuit utilizing the same.
2. Related Art
FIG. 18 is a schematical block diagram of a load drive circuit using a charge pumping circuit. In FIG. 18, the load drive circuit is provided with an N channel type MOS transistor 2, which has advantageous features such as low ON resistance for on-off control of a load 1. Specifically, an MOS transistor 2 is used as a high side switch, which has its drain connected to a power source +B and to a ground terminal via the load 1. In this case, to maintain the MOS transistor 2 in the turned-on condition, a gate control voltage must be set to a level higher than a supply voltage VB. For this reason, the gate control voltage is obtained by a booster circuit 3 with power supplied from the power source +B. Together with an oscillation circuit 4 with power supplied from the power source +B, the booster circuit makes up a charge pumping circuit 5. A concrete example of this charge pumping circuit 5 is given in FIG. 17.
As shown in FIG. 17, a booster circuit 3 comprises a power terminal Qin connected to a power source +B, a signal input terminal Qosc connected to output terminal of an oscillation circuit 4, an output terminal Qout connected to gate of an MOS transistor 2, and an earth terminal GND connected to a ground terminal. Also, the booster circuit 3 comprises inverter circuits 6, 7 and 8 connected between the power terminal Qin and the earth terminal GND, reverse-blocking diodes 9, 10 and 11 connected in forward direction and in series between the power terminal Qin and the output terminal Qout, a first capacitor 12 connected between cathode of the diode 9 and the output terminal of the inverter circuit 6, and a second capacitor 13 connected between cathode of the diode 10 and the output terminal of the inverter circuit 8. In this case, each of the inverter circuits 6 and 7 has its input terminal connected to the signal input terminal Qosc, and the output terminal of the inverter circuit 7 is connected to the input terminal of the inverter circuit 8.
In the charge pumping circuit 5 with the above arrangement, output of the inverter circuit 6 is alternately reversed between output voltage level (=VB) of the power source +B and the ground potential level in synchronization with oscillation output from the oscillation circuit 4. Also, output of the inverter circuit 8 has a phase by 180xc2x0 different from the phase of the output of the inverter circuit 6. During the period when output voltage of the inverter circuit 6 is turned to the ground potential level, electric power is charged to the first capacitor 12 from the power source +B via the diode 9. Thereafter, during the period when output voltage of the inverter circuit 6 is turned to the supply voltage VB and output voltage of the inverter circuit 8 is turned to the ground potential level, electric charge charged on the first capacitor 12 is shifted to the second capacitor 13 via the diode 10 and this operation is repeated. By this charge pumping operation, a voltage boosted to higher level than the supply voltage VB is outputted from the output terminal Qout.
In the conventional type circuit configuration as described above, a voltage corresponding to a difference between the supply voltage VB and the ground potential level (actually, a voltage lower by the voltage drop in forward direction of the diode 9) is applied on the first capacitor 12, and a voltage of about twice as high as the supply voltage VB is applied on the second capacitor 13. As a result, when fluctuation of the supply voltage VB inevitably occurs such as the case where the power source +B is a battery used on board of a vehicle, the voltage applied on the capacitors 12 and 13 is increased when the supply voltage VB is increased, and this adversely affects service life of the capacitors 12 and 13.
Further, in the conventional case as described above, output current of the booster circuit 3 is increased proportionally to the increase of the supply voltage VB. When it is tried to obtain an ability to meet the specification required on the output current, this requirement must be satisfied also under the condition where the supply voltage VB is decreased to the lowest operating voltage. For this reason, when the supply voltage VB is at normal level, the circuit inevitably consumes output current uselessly, and it is difficult to decrease this consumption of electric current.
To solve the above problems, it is an object of the present invention to provide a charge pumping circuit, by which it is possible to improve reliability on service life of capacitors for boosting provided in the circuit even under condition where the supply voltage fluctuates, and also it is possible to reduce the consumption current. The invention also provides a load drive circuit using the charge pumping circuit.
To attain the above object, the charge pumping circuit according to the present invention comprises a first capacitor to be charged intermittently from a power source and a second capacitor, to which electric charge charged on the first capacitor is shifted via reverse-blocking means for the purpose of boosting the supply voltage. There is provided a constant voltage generation circuit for generating a reference voltage maintained on a level lower by a fixed voltage than the supply voltage under normal condition, and charging operation to the first capacitor and the second capacitor is performed with the reference voltage as reference. By the arrangement as described above, when the supply voltage is increased, the reference voltage is also increased in response to it, and the relationship between the supply voltage and the reference voltage is maintained always at a constant level. As a result, the maximum value of the voltage applied on the first capacitor and the second capacitor is fixed on a certain fixed level. Specifically, the voltage applied on the first and the second capacitors is not increased in association with the increase of the supply voltage as in the conventional type system. Therefore, there is no adverse effect on service life of the first capacitor and the second capacitor, and this contributes to the improvement of reliability on the service life of the capacitors. Further, the booster circuit is operated between the supply voltage and the reference voltage, which is lower by a certain fixed voltage value than the supply voltage. As a result, operating condition of the booster circuit is maintained at a constant level regardless of the fluctuation of the supply voltage. For this reason, if it is determined in such manner that boosting ability of the booster circuit is in stabilized state to maintain a constant relationship between the reference voltage and the supply voltage, useless output current does not flow even when the supply voltage is increased, and this contributes to the reduction of consumption current.
According to a preferred aspect of the present invention, the reference voltage is outputted from a current amplifier circuit of high input impedance, and the reference voltage is very unlikely to be changed even when load current is changed. As a result, it is possible to prevent undesirable condition where maximum value of the voltage applied on the first capacitor and the second capacitor is unexpectedly changed.
According to another preferred aspect of the present invention, the current amplifier circuit comprises a voltage follower using an operational amplifier. As a result, stable operation with very few errors can be expected.
According to still another preferred aspect of the present invention, the current amplifier circuit comprises a Darlington transistor circuit. This contributes to simplification of the circuit design compared with the case where the voltage follower is used.
According to yet still another preferred aspect of the present invention, each of the first capacitor and the second capacitor comprises a plurality of unit capacitors connected in series. As a result, even when short-circuiting failure may occur in one of the unit capacitors, which make up the first and the second capacitors, boosting function by the first capacitor and the second capacitor can be maintained, and this contributes to the improvement of operation reliability.
According to still another preferred aspect of the present invention, each time the output voltage of the first switching circuit is changed to the reference voltage, charging current is supplied from the power source to the first capacitor via a first rectifying device. During the period when output voltage of the first switching circuit is changed to the reference voltage, electric charge charged on the first capacitor is shifted to the second capacitor via a second rectifying device in response to the change of the output voltage of the second switching circuit to the reference voltage. As a result, boosting operation of the supply voltage is carried out.
According to still another preferred aspect of the present invention, the first switching circuit and the second switching circuit are provided by inverter circuits, which comprise diverse circuit configuration, and this contributes to the improvement of the degree of freedom in circuit design.
In the load drive circuit according to the present invention, the charge pumping circuit with the above characteristics is used, and the load is driven by on-off control of an N channel MOS transistor, which has advantageous features such as low ON resistance. This makes it possible to supply high load current and to improve the reliability on service life.